1. Field
The embodiment discussed herein are directed to a servo frame interval correction method in a storage apparatus for correcting intervals between servo frames formed on a recording medium having a deviation from the center of rotation.
2. Description of Related Art
A magnetic head positioning control method controlled by a conventional magnetic disk drive will be disclosed with reference to FIG. 1.
A manipulated variable u 101 is input to an actual mechanics 102 as a driving electric power value of a power amplifier for a voice coil motor. Then, a demodulated position 103 is an actual position of the servo frame read with the magnetic moved by the actual mechanics 102. Conventionally, the magnetic disk drive calculates a position and velocity of a subsequent servo frame by adding a control matrix Z−1 to a present position of the magnetic head (calculated position 104), to a present velocity of the magnetic head (calculated velocity 105), and to a calculated external force 106, which is a bias component, respectively. The control matrix Z−1 will be described in detail later.
However, errors will be accumulated gradually with this calculation method.
Thus, the magnetic disk drive calculates a differential between the calculated position 104 and the demodulated position 103 as an estimation error 107. Then the estimation error 107 is added to a coefficient of the calculated position Lp, a coefficient of the calculated velocity Lv, and a coefficient of the calculated external force Lb, respectively. Then, the position of the servo frame to be detected subsequently (estimated position 108) is calculated by adding up a summation of the calculated error 107 and the coefficient Lp and a summation of the control matrix and the calculated position 104. Likewise, estimated velocity 109 on reaching the subsequent servo frame, and an estimated external force 110 are calculated by the magnetic disk drive.
Using proportional-integral-derivative control (PID control), a manipulated variable u 111 is determined by adding up a summation of the estimated position 108 and a proportional term Kp, and a summation of the estimated velocity 109 and a derivative term Kv. The magnetic head is controlled according to the manipulated variable u 111 determined above. The estimated external force 110 is not taken into consideration in the determination of the manipulated variable u because the estimated external force is processed outside of the PID control.
Thus, the magnetic head is correctly positioned according to the manipulated variable u. Then, when the process to search for the subsequent servo frame switches, an estimated position and an estimated velocity of the subsequent servo frame are derived by substituting the estimated position for a calculated position, the estimated velocity for a calculated velocity, and the estimated external force for a calculated external force.
A digital signal processor (DSP) may detect a demodulated position and a current applied to a magnetic head incorporated in the magnetic disk drive via a power amplifier by the DSP. There are two calculations to obtain the velocity of the magnetic head: one is to obtain the velocity from a differential between positions; and the other is to obtain the velocity by integrating an acceleration based on the current. In modern control methods, the velocity is typically derived by integrating the acceleration.
The control of the magnetic head position described above is designed on an assumption that servo frames are obtained at equal-spaced intervals.
There has been a control apparatus employing a conventional technique relating to the present technique for positioning a magnetic head, correcting a variation of a servo signal attributed to a run-out error caused by decentering of a disk.
Further, a technique with which position of a head incorporated in a disk drive is detected by synchronizing detailed position information has been disclosed. Additionally, a technique for reducing unnecessary disk format margin is employed to increase disk format efficiency. Reference documents are Japanese Laid-open Patent Publication Nos. 1999-353831, 1998-507027, 2006-12350.
However, for a magnetic disk drive employing a stack servo track writer (SSTW), servo frames are infinitesimally deviated from the center of rotation due to a mismatch between the rotation centers of the SSTW and a magnetic disk drive in which a magnetic disk is incorporated.
Thus, the magnetic head traces an elliptic track, which causes the lengths of the servo frames to vary. The conventional control system is designed with an assumption that position signals are detected at equal-spaced intervals. However, the intervals between the servo frames become irregular because of the elliptic track. The variation in the servo frame detection time intervals is not a problem when seek velocity is slow. However, as the seek velocity becomes faster, the variation in the servo frame detection time intervals affects seeking accuracy to a greater degree.
Given that the magnetic head moves 300 tracks from one servo frame to another servo frame, and the magnetic head is positioned 30 mm away from the center of rotation, and the decentering of the recording medium is 30 um, the error is 0.1% when converted to time. Where the head moves over 300 tracks, the error is 0.3 tracks, which is insignificant. However, where the errors are accumulated in the control system, positioning a target is difficult due to the positioning errors. As a result the seek time is increased which leads to reduced access times and a deterioration in the performance of the magnetic disk drive.
This is thought to be because of the problem of servo frame variations, in the form of calculated errors that arise due to the difference between the estimated position and the actual position of the magnetic head, included in the control system.
The embodiment of the present invention is disclosed to solve the problem mentioned above.
Accordingly, an object of the embodiment is to provide a servo frame interval correction apparatus, a storage apparatus, and a servo frame interval correction method for correcting estimated positions (or calculated positions).